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  Section: Plant Nutrition » Micronutrients » Boron
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Diagnosis of Boron Status in Plants

Historical Information
  Determination of Essentiality
  Functions in Plants
    - Root Elongation and Nucleic Acid Metabolism
    - Protein, Amino Acid, and Nitrate Metabolism
    - Sugar and Starch Metabolism
    - Auxin and Phenol Metabolism
    - Flower Formation and Seed Production
    - Membrane Function
Forms and Sources of Boron in Soils
  Total Boron
  Available Boron
  Fractionation of Soil Boron
  Soil Solution Boron
  Hydrated Boron Minerals
Diagnosis of Boron Status in Plants
  Deficiency Symptoms
    - Field and Horticultural Crops
    - Other Crops
  Toxicity Symptoms
    - Field and Horticultural Crops
    - Other Crops
Boron Concentration in Crops
  Plant Part and Growth Stage
  Boron Requirement of Some Crops
Boron Levels in Plants
Soil Testing for Boron
  Sampling of Soils for Analysis
  Extraction of Available Boron
    - Hot-Water-Extractable Boron
    - Boron from Saturated Soil Extracts
    - Other Soil Chemical Extractants
  Determination of Extracted Boron
    - Colorimetric Methods
    - Spectrometric Methods
Factors Affecting Plant Accumulation of Boron
  Soil Factors
    - Soil Acidity, Calcium, and Magnesium
    - Macronutrients, Sulfur, and Zinc
    - Soil Texture
    - Soil Organic Matter
    - Soil Adsorption
    - Soil Salinity
  Other Factors
    - Plant Genotypes
    - Environmental Factors
    - Method of Cultivation and Cropping
    - Irrigation Water
Fertilizers for Boron
  Types of Fertilizers
  Methods and Rates of Application

Boron deficiency in crops is more widespread than deficiency of any other micronutrient. This phenomenon is the chief reason why numerous reports are available on boron deficiency symptoms in plants. Because of its immobility in plants, boron deficiency symptoms generally appear first on the younger leaves at the top of the plants. This occurrence is also true of the other micronutrients except molybdenum, which is readily translocated.

Boron toxicity symptoms are similar for most plants. Generally, they consist of marginal and tip chlorosis, which is quickly followed by necrosis (58). As far as boron toxicity is concerned, it occurs chiefly under two conditions, owing to its presence in irrigation water or owing to accidental applications of too much boron in treating boron deficiency. Large additions of materials high in boron, for example, compost, can also result in boron toxicity in crops (59,60). Boron toxicity in arid and semiarid regions is frequently associated with saline soils, but most often it results from the use of high-boron irrigation waters. In the United States, the main areas of high-boron waters are along the west side of the San Joaquin and Sacramento valleys in California (61).

Boron does not accumulate uniformly in leaves, but typically concentrates in leaf tips of monocotyledons and leaf margins of dicotyledons, where boron toxicity symptoms first appear. In fact although leaf tips may represent only a small proportion of the shoot dry matter, they can contain sufficient boron to substantially influence total leaf and shoot boron concentrations. To overcome this problem, Nable et al. (62) recommended the use of grain in barley for monitoring toxic levels of boron accumulation. The main difficulty in using cereal grain for determining boron levels is the small differences in the grain boron concentration as obtained in response to boron fertilization (63). Low risk of boron toxicity to rice in an oilseed rape (Brassica napus L.)–rice (Oryza sativa L.) rotation was attributed to the relatively high boron removal in harvested seed, grain, and stubble, and the loss of fertilizer boron to leaching (64). Boron toxicity symptoms in zinc-deficient citrus (Citrus aurantium L.) could be mitigated with zinc applications. This finding is of practical importance as boron toxicity and zinc deficiencies are simultaneously encountered in some soils of semiarid zones.


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